- Interaction of oxides of 3d transition metals with boron
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Differential thermal analysis and X-ray phase analysis were used to investigate the preparation conditions and composition of products of the borothermal reduction of oxides of 3d transition metals (titanium, vanadium, chromium, manganese, iron, cobalt, n
- Solov'ev,Makarov,Meshchaninova,Ugai
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- Crystal structure refinement and bonding patterns of CrB4: A boron-rich boride with a framework of tetrahedrally coordinated B atoms
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Crystals of chromium tetraboride, a recently proposed candidate superhard material, have been grown for the first time to allow for a first structure refinement of the compound [orthorhombic, space group Immm (No. 71), a = 474.82(8) pm, b = 548.56(8) pm, and c = 287.17(4) pm, R value (all data) = 0.018]. The previously proposed structure model is confirmed, and accurate interatomic distances are presented for the first time. First-principles electronic structure calculations emphasize the unique framework of three-dimensionally linked B atoms that are tetrahedrally coordinated and carry a slightly negative charge. All B-B bonding is of the 2-center 2-electron type. CrB4 is metallic with a pseudogap at the Fermi level.
- Knappschneider, Arno,Litterscheid, Christian,Kurzman, Joshua,Seshadri, Ram,Albert, Barbara
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- Electronic structure and magnetic properties of transition metal diborides
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The temperature dependencies of the magnetic susceptibility χ and its anisotropy Δ χ = χ{norm of matrix} - χ⊥ were measured for the hexagonal single crystalline TB2 compounds (T = Sc, Ti, Zr, Hf, V and Cr) in the temperatu
- Grechnev,Fedorchenko,Logosha,Panfilov,Svechkarev,Filippov,Lyashchenko,Evdokimova
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- Preparation of borides in Nb-B and Cr-B systems by combustion synthesis involving borothermic reduction of Nb2O5 and Cr2O3
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An experimental study on the preparation of metal borides in the Nb-B and Cr-B systems was conducted by self-propagating high-temperature synthesis (SHS) involving the reduction of Nb2O5 and Cr2O3 by amorphous boron. The starting stoichiometry of the reactant compact was shown to make a great impact on the combustion behavior and the phase composition of the final product. For the powder compacts of Nb2O5 and boron, self-sustaining combustion was performed under a molar ratio of B/Nb2O5 between 5 and 10, but complete reduction of Nb2O5 was achieved when B/Nb2O5 ≥ 8. Partial reduction of Nb2O5 caused a decrease in the combustion temperature and velocity, and was responsible for the presence of NbO2 in the final products. For the samples with stoichiometry of 6 ≤ B/Nb2O5 ≤ 8, three boride phases NbB, Nb3B4, and NbB2 were synthesized. An increase in the boron content up to B/Nb2O5 = 8.5-10 resulted in not only full reduction of Nb2O5, but also formation of single-phase NbB2. On the other hand, the SHS process involving Cr2O3 and boron was feasible for the powder compacts of 4 ≤ B/Cr2O3 ≤ 9, wherein the highest combustion temperature and the fastest reaction front were observed in the compact with B/Cr2O3 = 6. During combustion Cr2O3 was fully reduced, leading to the formation of three borides Cr5B3, CrB, and CrB2 in either monolithic or composite form. With a boron content more than the stoichiometric amount, the powder compacts of B/Cr2O3 = 4, 5, and 9 yielded single-phase Cr5B3, CrB, and CrB2, respectively.
- Yeh,Wang
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- A new class of CVD precursors to metal borides: Cr(B3H 8)2 and related octahydrotriborate complexes
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Treatment of CrCl3 with sodium octahydrotriborate, NaB3H8, affords a thermally unstable purple liquid thought to be a chromium(III) hydride of stoichiometry CrH(B3H8)2. This hydride converts rapidly at room temperature to the chromium(II) complex Cr(B3H8)2, which adopts a square-planar structure in which four hydrogen atoms form the coordination sphere of the chromium atom. This chromium(II) species forms six-coordinate Lewis base adducts Cr(B3H8)2L2, where L is Et2O, thf, or PMe3; the first two of these adopt trans geometries, whereas the latter is cis. Volatile Cr(B3H8)2 is the first homoleptic transition metal complex of the octahydrotriborate anion, and it readily forms CrB2 thin films by CVD at 250 °C. Copyright
- Goedde, Dean M.,Girolami, Gregory S.
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- Syntheses and Sinterings of Chromium Borides via Solid-State Reactions and Mechanochemical Processes of Chromium-Amorphous
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As a basic study for the development of chromium boride materials which are expected as refractory materials or hard materials, syntheses and sinterings of the chromium borides via solid state reactions and mechanochemical processes of chromium-amorphous boron mixed powders were studied.The single phases of Cr2B, CrB and CrB2 were obtained by the solid stat e reaction in the temperatures of 1400-1500°C. These borides wereso unsinterable that could be consolidated by pressure sintering only. The chromium boride of the highest relative density and hardness among t he ceramics obtained was CrB2. By mechanochemical processes, the single phases of CrB2 and CrB were obtained by milling of Cr-B(molar ratio,1:2)and Cr-B(1:1) mixed powders by using a planetary ball mill for 20-40 h, followed by annealing at 900 and 1000°C, respectively. As-milled Cr-B(1:1) mixed powders were consolidated by both of pressureless and p ressure sinterings. These are a reaction sintering because the sinterings are accelerated by the exothermic reaction heat of the unreacted Cr-B in the as-milled mixed powders. The unsinterable powder could be consolidated by pressureless sintering via mechanochemical process. At pressuresintering via mechanochemical process, the sintering temperature lowere d and the hardness of ceramics obtained increased.
- Iizumi, Kiyokata,Kudaka, Katsuya
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- Mechanochemical Synthesis of CrB2-MoB2 Composite Powder
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Ball-milling of elemental powder mixtures without external heat application has been utilized as a process for the mechanochemical synthesis of high melting compounds. The mixtures of chromium, molybdenum and amorphous boron elemental powders with atomic ratios Cr:Mo:B=(1-x) : x : 2(x=0-1.0) were comminuted for 10-40 h in a planetary ball-mill, and single hexagonal phase (Cr1-xMoxB2) was formed after 40 h of milling using tungsten carbide balls. By annealing at 900-1400°C, a single hexagonal phase was still observed at the compositions of x=0-0.6. At x=0.8 and 1.0, the hexagonal phase was observed to be partly transformed to rhombohedral phase by annealing at the temperatures above 1300 and 1000°C, respectively. The variation in the lattice parameters of the hexagonalphase with x was estimated to differ from Vegard's law because of a ten dency toward superlattice formation. The rhombohedral phase was estimated to be Mo2B5-x type by the determination of lattice parameters.
- Sasaki, Takeshi,Iizumi, Kiyokata,Kudaka, Katsuya,Okada, Shigeru
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- THERMAL EXPANSION STUDIES ON THE GROUP IV-VII TRANSITION METAL DIBORIDES.
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The thermal expansions of the group IV-VII transition metal diborides were studied with the aid of X-ray powder diffraction. The diborides were studied over the temperature range 298 - 1500 K. All the diborides except for CrB//2 display larger thermal expansion coefficients in the c direction than in the a direction. The expansion coefficients in the c direction decrease with increasing radius of the metal atom, a fact which can be correlated to an increase in metal-boron bond strength. The thermal expansion coefficient in the a direction changes very little with the size of the metal radius, owing to the fact that the bonding strength in the basal plane is determined by the strong B-B bonds within the boron layer.
- Loennberg
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p. 145 - 156
(2008/10/08)
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